About finding the number of permutations
Let there be n pairs of 2*n students : (1, 2) , (3, 4), (5, 6)
... (2n-1 , 2n). We want to find the number of arrangements of
students which the pair are not adjacent. In other words, for (2*i)
th student, the (2*i -1) th student should not be in his front or
back.
For example, think of case of n=2. In this case, (1, 4, 3, 2) is
not appropriate for...